The present invention relates to a system for processing audio frequency information, particularly speech, for frequency modulation and to an FM transmitter including said system.
In land mobile FM transmitters, there are mandatory requirements for the frequency response and overload performance of such transmitters. In order to meet these requirements an audio frequency modulating signal is processed so that when it has a relatively low level the transmitter will not have to go through its full modulation capability, but when it has a high level then the response is limited in order to maintain channel integrity. Generally, the frequency response of the transmitted signal has a pre-emphasis characteristic which provides various advantages in terms of noise in the complete transmitter/receiver system. The pre-emphasis characteristic emphasizes the modulating speech signal up to substantially 3 kHz and then there is a filter requirement which means that the level must shut down or roll-off fairly sharply after 3 kHz. FIG. 1 of the accompanying drawings shows three curves 10, 12, 14 representing the pre-emphasis characteristics of modulating signals having low, intermediate and high levels, L, respectively. The horizontal broken line 16 indicates the maximum peak voltage and corresponds to the maximum peak deviation frequency of the transmitter. When comparing the characteristic curves 12 and 14, it is evident that the frequency response of the transmitter has to change as the modulating signal level approaches the maximum peak deviation (denoted by the broken line 16). This change is from a pre-emphasized characteristic, curve 12, to a flat, limiting response such that at no audio frequency is the maximum peak deviation frequency exceeded. The audio signal processing curcuitry has to be arranged so that it passes low and intermediate levels of modulating signals substantially undistorted but clips or limits high levels of modulating signal such that regardless of frequency it does not exceed the maximum peak voltage, the broken line 16 in FIG. 1. The voltage generated by the audio frequency processor corresponds to the instantaneous frequency deviation of the transmitter and the absolute maximum output voltage of the processor corresponds to the peak system deviation the transmitter is capable of producing.
A known audio processing circuit is illustrated in FIG. 2 of the accompanying drawings while FIG. 3 illustrates diagrammatically the frequency characteristic of each stage of the circuit shown in FIG. 2. In FIG. 2, the audio processing circuit comprises a transducer in the form of a microphone 18 which is connected to a pre-emphasis device 20. The output from the device 20 is applied to a clipper circuit 22 which clips the pre-emphasized signal if it exceeds a predetermined level. The output from the clipper circuit 22 is applied to a low-pass filter 23 which has a sharp roll-off above 3 kHz. In order to obtain the desired roll-off characteristic, the low-pass filter 23 comprises a Chebyschev filter 24 whose output is coupled to a Butterworth filter 26. The output from the low-pass filter 23 is derived from a terminal 28 and is used to frequency modulate a transmitter.
Referring to FIG. 3 the frequency characteristics of each part of the circuit in FIG. 2 are shown, and for convenience of identification, each characteristic is referenced with the number of the part of circuit with a suffix A. Thus, the microphone 18 has a characteristic 18A which is substantially flat between 300 Hz and 5 kHz. As shown, the pre-emphasis characteristic 20A increases substantially linearly between 300 Hz and 3 kHz whereas the clipper characteristic 22A is substantially flat over this frequency range and corresponds to the maximum peak voltage (see the broken line 16 in FIG. 1). The characteristic 24A of the Chebyschev filter 24 rises non-linearly between 300 Hz and 3 kHz after which it rolls-off fairly rapidly. In contrast, the characteristic 26A of the Butterworth filter rolls-off steadily between 300 Hz and 3 kHz and more sharply thereafter. The effect of constructing the low-pass filter 23 from the filters 24 and 26 is shown by the overall resultant filter characteristic 23A in FIG. 4 of the accompanying drawings. The resultant characteristic 23A is substantially flat up to 3 kHz and then rolls-off sharply.
The known circuit arrangement of FIG. 2 has a disadvantage that the maximum peak output level for an undistorted signal, for example a sine wave, is typically between 60% and 70% of the high level maximum peak output level. The reasons for this can be understood from a consideration of FIG. 5 of the accompanying drawings. In FIG. 5, the broken line sine wave represents the maximum peak signal which can be processed by the circuit shown in FIG. 2 without being distorted, the output peak amplitude being designated by the letter P. The frequency of this signal is 1 kHz. The waveforms shown in full lines represent a signal level which is sufficiently high that it becomes limited during the audio processing. The input signal is shown in diagram A of FIG. 5. Diagram B is the waveform of the signal after pre-emphasis and clipping. Diagram C shows the waveform at the output of the Chebyschev filter 24. Because the input thereto is a stepped waveform, the rising edge of the waveform produces an overshoot and ringing at the cut-off frequency of the filter, that is at 3 kHz. This waveform is passed substantially unchanged by the Butterworth filter 26, Diagram D. In the frequency modulator the overshoot is treated as the maximum peak output level, designated by the letter Q, which causes the maximum frequency deviation. In most mobile radio equipment, the peak undistorted output (P) is typically between 60% and 70% of the maximum peak output level.